/**************************************************************************/ /* safe_list.h */ /**************************************************************************/ /* This file is part of: */ /* GODOT ENGINE */ /* https://godotengine.org */ /**************************************************************************/ /* Copyright (c) 2014-present Godot Engine contributors (see AUTHORS.md). */ /* Copyright (c) 2007-2014 Juan Linietsky, Ariel Manzur. */ /* */ /* Permission is hereby granted, free of charge, to any person obtaining */ /* a copy of this software and associated documentation files (the */ /* "Software"), to deal in the Software without restriction, including */ /* without limitation the rights to use, copy, modify, merge, publish, */ /* distribute, sublicense, and/or sell copies of the Software, and to */ /* permit persons to whom the Software is furnished to do so, subject to */ /* the following conditions: */ /* */ /* The above copyright notice and this permission notice shall be */ /* included in all copies or substantial portions of the Software. */ /* */ /* THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, */ /* EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF */ /* MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. */ /* IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY */ /* CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, */ /* TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE */ /* SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE. */ /**************************************************************************/ #ifndef SAFE_LIST_H #define SAFE_LIST_H #include "core/os/memory.h" #include "core/typedefs.h" #include <atomic> #include <functional> #include <type_traits> // Design goals for these classes: // - Accessing this list with an iterator will never result in a use-after free, // even if the element being accessed has been logically removed from the list on // another thread. // - Logical deletion from the list will not result in deallocation at that time, // instead the node will be deallocated at a later time when it is safe to do so. // - No blocking synchronization primitives will be used. // This is used in very specific areas of the engine where it's critical that these guarantees are held. template <class T, class A = DefaultAllocator> class SafeList { struct SafeListNode { std::atomic<SafeListNode *> next = nullptr; // If the node is logically deleted, this pointer will typically point // to the previous list item in time that was also logically deleted. std::atomic<SafeListNode *> graveyard_next = nullptr; std::function<void(T)> deletion_fn = [](T t) { return; }; T val; }; static_assert(std::atomic<T>::is_always_lock_free); std::atomic<SafeListNode *> head = nullptr; std::atomic<SafeListNode *> graveyard_head = nullptr; std::atomic_uint active_iterator_count = 0; public: class Iterator { friend class SafeList; SafeListNode *cursor = nullptr; SafeList *list = nullptr; Iterator(SafeListNode *p_cursor, SafeList *p_list) : cursor(p_cursor), list(p_list) { list->active_iterator_count++; } public: Iterator(const Iterator &p_other) : cursor(p_other.cursor), list(p_other.list) { list->active_iterator_count++; } ~Iterator() { list->active_iterator_count--; } public: T &operator*() { return cursor->val; } Iterator &operator++() { cursor = cursor->next; return *this; } // These two operators are mostly useful for comparisons to nullptr. bool operator==(const void *p_other) const { return cursor == p_other; } bool operator!=(const void *p_other) const { return cursor != p_other; } // These two allow easy range-based for loops. bool operator==(const Iterator &p_other) const { return cursor == p_other.cursor; } bool operator!=(const Iterator &p_other) const { return cursor != p_other.cursor; } }; public: // Calling this will cause an allocation. void insert(T p_value) { SafeListNode *new_node = memnew_allocator(SafeListNode, A); new_node->val = p_value; SafeListNode *expected_head = nullptr; do { expected_head = head.load(); new_node->next.store(expected_head); } while (!head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ new_node)); } Iterator find(T p_value) { for (Iterator it = begin(); it != end(); ++it) { if (*it == p_value) { return it; } } return end(); } void erase(T p_value, std::function<void(T)> p_deletion_fn) { Iterator tmp = find(p_value); erase(tmp, p_deletion_fn); } void erase(T p_value) { Iterator tmp = find(p_value); erase(tmp, [](T t) { return; }); } void erase(Iterator &p_iterator, std::function<void(T)> p_deletion_fn) { p_iterator.cursor->deletion_fn = p_deletion_fn; erase(p_iterator); } void erase(Iterator &p_iterator) { if (find(p_iterator.cursor->val) == nullptr) { // Not in the list, nothing to do. return; } // First, remove the node from the list. while (true) { Iterator prev = begin(); SafeListNode *expected_head = prev.cursor; for (; prev != end(); ++prev) { if (prev.cursor && prev.cursor->next == p_iterator.cursor) { break; } } if (prev != end()) { // There exists a node before this. prev.cursor->next.store(p_iterator.cursor->next.load()); // Done. break; } else { if (head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ p_iterator.cursor->next.load())) { // Successfully reassigned the head pointer before another thread changed it to something else. break; } // Fall through upon failure, try again. } } // Then queue it for deletion by putting it in the node graveyard. // Don't touch `next` because an iterator might still be pointing at this node. SafeListNode *expected_head = nullptr; do { expected_head = graveyard_head.load(); p_iterator.cursor->graveyard_next.store(expected_head); } while (!graveyard_head.compare_exchange_strong(/* expected= */ expected_head, /* new= */ p_iterator.cursor)); } Iterator begin() { return Iterator(head.load(), this); } Iterator end() { return Iterator(nullptr, this); } // Calling this will cause zero to many deallocations. bool maybe_cleanup() { SafeListNode *cursor = nullptr; SafeListNode *new_graveyard_head = nullptr; do { // The access order here is theoretically important. cursor = graveyard_head.load(); if (active_iterator_count.load() != 0) { // It's not safe to clean up with an active iterator, because that iterator // could be pointing to an element that we want to delete. return false; } // Any iterator created after this point will never point to a deleted node. // Swap it out with the current graveyard head. } while (!graveyard_head.compare_exchange_strong(/* expected= */ cursor, /* new= */ new_graveyard_head)); // Our graveyard list is now unreachable by any active iterators, // detached from the main graveyard head and ready for deletion. while (cursor) { SafeListNode *tmp = cursor; cursor = cursor->graveyard_next; tmp->deletion_fn(tmp->val); memdelete_allocator<SafeListNode, A>(tmp); } return true; } ~SafeList() { #ifdef DEBUG_ENABLED if (!maybe_cleanup()) { ERR_PRINT("There are still iterators around when destructing a SafeList. Memory will be leaked. This is a bug."); } #else maybe_cleanup(); #endif } }; #endif // SAFE_LIST_H